1. Field of the Invention
The present invention relates to circular hole cutting devices which are used for making holes for passing electric wires or water pipes, for instance, in ceilings, walls or floors in a building or house.
2. Description of the Background Art
FIG. 14 shows an example of a conventionally known circular hole cutting device 100. This circular hole cutting device 100 comprises a cylindrical attachment 101, a guide rod 102 passing through the cylindrical attachment 101 at right angles to but at a slight distance from its central axis, and a stopper 103 which is screwed into a curved wall of the cylindrical attachment 101 at right angles to the guide rod 102 in such a way that an extreme end of the stopper 103 comes into contact with the guide rod 102.
The cylindrical attachment 101 is securely attached to an electric power tool 105 as one end of the cylindrical attachment 101 is fitted over a far end of the electric power tool 105 as illustrated. A bit 104 which is turned at high speeds about a central axis of the electric power tool 105 is detachably fitted to its far end. The physical size of the cylindrical attachment 101 is determined such that the bit 104 protrudes to the outside of the cylindrical attachment 101 when the cylindrical attachment 101 is mounted on the electric power tool 105.
The guide rod 102 has a pivot shaft 102a at one end formed by bending a terminal part of the guide rod 102 at right angles to its main part. The extreme end of the pivot shaft 102a is sharply pointed.
The circular hole cutting device 100 thus constructed is used as follows. First, a small center hole is made by the bit 104 at the center of a circular hole to be formed in a ceiling, for instance, operating the electric power tool 105. Then, with the pivot shaft 102a fitted into the center hole, the electric power tool 105 is turned about the pivot shaft 102a while turning the bit 104 at a high speed. As the bit 104 moves in a circle all around the pivot shaft 102a, the ceiling is cut by the bit 104 so that the circular hole for passing an electric wire or a water pipe, for instance, is made therein.
In the conventional circular hole cutting device 100, the cylindrical attachment 101 is mounted on the electric power tool 105 as described above. It is to be noted, however, that different types of electric power tools have different shapes. Accordingly, one problem of the conventional circular hole cutting device 100 is that it lacks flexibility in use due to the need to prepare various cylindrical attachments specially designed for use with different types of electric power tools.
Furthermore, when turning the electric power tool 105 about the pivot shaft 102a, it is necessary to continuously turn the electric power tool 105 about its own axis so that an operator has to change his or her working posture. This poses another problem of the conventional circular hole cutting device 100 that it involves low labor efficiency.
This invention is intended to solve the aforementioned problems of the prior art. It is therefore an object of the invention to provide a circular hole cutting device featuring high flexibility in use with adaptability to various types of power tools and ease of operation.
According to a principal feature of the invention, a circular hole cutting device which makes a circular hole in an object when turned about a center hole formed in the object comprises a bit holder which can be attached to a drive shaft of an electric power tool in such a way that the bit holder with a bit attached to a far end rotates together with the drive shaft on a common axis, a relatively rotatable part fitted to the bit holder relatively rotatably about the axis of the drive shaft, and a pivot shaft which can be inserted into the center hole formed by the bit, wherein the pivot shaft connected to the relatively rotatable part is located at a position offset from the bit in its radial direction.
With this circular hole cutting device, it is possible to form a circular hole whose radius corresponds to the distance between the bit and the pivot shaft in the object by inserting the pivot shaft in the center hole, activating the electric power tool to turn the bit at a high speed, and slowly turning the electric power tool about the pivot shaft with the bit passing through the object so that the bit turning at the high speed cuts the object along a circular path.
The drive shaft of the electric power tool usually has a chuck complying with industrially standardized dimensional specifications. Therefore, the circular hole cutting device of this invention can be connected to practically any electric power tool regardless of its manufacturer or type, so that the circular hole cutting device provides great flexibility in application.
Furthermore, since the relatively rotatable part and the bit holder are connected relatively rotatably with each other, the relatively rotatable part does not rotate together with the drive shaft of the electric power tool even when the relatively rotatable part is turned about the pivot shaft. Accordingly, it is not necessary for an operator to change his or her working posture to turn the circular hole cutting device about its own axis when making a hole in a ceiling, for instance. This would help increase labor efficiency in hole-cutting operation.